1. Field of the Invention
This invention relates to an optical information reading and reproducing apparatus which records and reproduces onto and from an optical information recording medium pseudo DC free codes generated on the basis of non-DC-free code information.
2. Description of the Related Art
Various methods of recording information onto an optical information recording medium such as an optical disk have been proposed.
For example, the existing optical information recording and reproducing apparatuses using 130-mm or 90-mm ISO standardized optical disks have been using a method of recording information onto an optical information recording medium by assigning information to the distance between pits by pit-position recording techniques. To double the memory capacity of the 130-mm ISO standardized optical disk, a second-generation optical information recording and reproducing apparatus has been proposed which employs the MCAV system that records information at almost equal recording density from the innermost to the outermost circumference. Even the second-generation optical information recording and reproducing apparatus uses a method of recording information onto an optical information recording medium by assigning information to the distance between pits by pit-position recording techniques.
In contrast, to triple the memory capacity of optical disks, mark-edge recording techniques have been proposed which assign information to the length of the pit, since it is difficult to increase the recording density by pit-position recording techniques.
With pit-position techniques, information exists between peak points of the reproduced signal from the recording medium. Thus, by obtaining the first-order 10 differential of the reproduced signal and then slicing its zero-crossing point, the information can be reproduced. Specifically, even when the recording information is modulated in 2-7 modulation codes, not DC-free codes, as shown in FIG. 11A, the resulting pit-position recording signal is a signal corresponding to "0" and "1" in the 2-7 modulation code system, as shown in FIG. 11B. Thus, the pit-position recording pits are formed as shown in FIG. 11C, which therefore introduces no problem to the reproduction of information at all.
In the case of mark-edge recording techniques proposed to increase the recording density, however, the resulting mark-edge recording signal is a signal corresponding to the change from "0" to "1" in the 2-7 modulation code system, so that the mark-edge recording pits are as shown in FIG. 11E. Specifically, because information is assigned to the distance between edges of the reproduced signal from the recording medium, accurate reproduction of information from the reproduced signal from the recording medium can be achieved by obtaining the second-order differential of the reproduced signal and then slicing its zero-crossing point. The second-order differential technique wherein the reproduced signal is passed through a differentiator twice has an S/N disadvantage in that it amplifies noises in high-frequency bands.
A level-slicing method as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-45470 can be considered to be another method of reproducing the information recorded by mark-edge recording techniques. In this case, when recording information is modulated in 2-7 or 1-7 modulation codes, not DC-free codes, the level-slicing method cannot accurately sense the edges of the reproduced signal from the recording medium, since the slicing level varies with the pattern of the reproduced signal due to AC coupling in the reproduction system.
To overcome this drawback, there has been proposed a method which, in the case of optical disks complying with a proposal for 130-mm ECMA standardization (ECMA/TC31/92/36) using a method of recording by assigning information to the length of the pit by mark-edge recording techniques, obtains the DSV (digital sum value) of 1s and 0s of the recording pits in the data portion before and after the resink pattern of the data area on the optical disk (i.e., obtaining the sum of 1s and that of 0s), and selecting and recording two types of resink patterns, depending on whether the number of 1s is larger than that of 0s. Then, the polarity of the recording pattern in the following data section is reversed to form a pseudo DC-free code, which is then recorded. In a reproducing operation, the two types of resink patterns are sensed and it is judged whether the following data is reversed or not. Then, the signal is further processed to reproduce the data.
In the proposal for 130 mm ECMA standardization, recording patterns are converted into pseudo DC-free codes on the basis of two types of sink patterns. However, it is virtually difficult to convert the recording data pattern into DC free codes on the basis of only two types of sink patterns. As a result, there is no guarantee that the DSV of 1s will be equal to the DSV of 0s. Therefore, it is impossible to basically overcome the problem encountered when a method of modulating recording information is not based on DC-free codes.